(1) Field of the Invention
The present invention relates to controlling the output power of a multistage radio frequency power amplifier by adjusting the power supply voltage level applied to the final stages of the power amplifier.
(2) Description of the Prior Art
In recent years, worldwide demand for wireless cellular communications has increased dramatically. Radiotelephones manufactured to meet this burgeoning demand must adhere to standards such as the Global System for Mobile Communications (GSM) standard. Another standard, the Digital Cellular System (DCS) standard is based on GSM, but is directed towards higher cell density and lower power. A third standard, Personal Communications Services (PCS), is a xe2x80x9ccatch allxe2x80x9d for many digital cellular systems, including GSM, operating in North America. These standards all require precise output power control over a large dynamic range in order to prevent a transmitter located in one cell from interfering with the reception of transmissions from other transmitters in neighboring cells.
A key component common to all radiotelephones is a radio frequency (RF) power amplifier. In modern digital radiotelephones, power amplifiers receive as input a frequency or phase modulated radio frequency carrier. The radio frequency carrier is what xe2x80x9ccarriesxe2x80x9d digital information such as digitized voice or data to a cellular base station. Before reaching the power amplifier, the RF carrier is too weak to be received by a cellular base station. Therefore it is the function of the power amplifier to boost the power of the RF carrier to a level sufficient for reception by a cellular base station.
Unfortunately, a simple single fixed power level will not work within a cellular network. Mobile users transmitting while traveling through multiple cells at a single fixed high power setting would overwhelm several cellular base stations. In contrast, a mobile user transmitting at a low power setting would result in unreliable short-range communication with perhaps a single cellular base station. To overcome this problem, engineers have designed radiotelephones with power amplifiers having multiple adjustably selectable power levels.
Accurately and efficiently selecting and controlling output power delivered by an RF amplifier remains a formidable task. For example, prior art systems sample output power by diverting a portion of their output power through the use of expensive components such as directional couplers. The diverted power is wasted, resulting in inefficiency reducing battery life and talk time.
Furthermore, most prior art systems also detect the RF power sample with a peak power diode detector circuit used to rectify and sense forward power. Through the rectification process, there is some squaring of the shape of the output power waveform. This squaring leads to higher harmonic content. The higher harmonic content requires additional and costly filtering because harmonic frequencies must be suppressed in order to comply with international communication regulations. Beyond detection, various other components are employed to compare a reference power level to the detected RF power sample. These components include buffers, attenuators, and passives, such as resistors.
Ultimately, a bias control circuit adjusts the gain of several amplifier stages to adjust the output power to an appropriate level. Generally, prior art bias control circuits involve substantial complexity due to large variations in power control loop bandwidth. Most often, prior art power control systems dedicate costly Application Specific Integrated Circuits (ASICs) to provide complex bias adjustments necessary to hold selected discrete power levels.
Another problem faced by conventional amplifier architectures is that of power control loop stability. In prior art systems, the gain varies widely across different power levels. It is common to find gain varying tenfold on a decibel scale over a full range of power levels. Amplifier gain is often referred to as control slope when considered as a control variable.
Whenever a power control signal, commonly referred to as an adjustable power control signal or APC, is applied to bias control circuitry, a given amplifier gain should be established for a given APC voltage. Unfortunately, highly nonlinear control slopes inherent in prior art systems are constantly changing due to external influences such as power supply fluctuations, temperature variations and output load changes. As a result of highly nonlinear and inconsistent control slope, it is difficult to design the proper control loop bandwidth to maintain control loop stability over all control slope regions. This results in increased design cycles, resulting in increased time to market.
In GSM radiotelephones, the adjustable power control signal must comply with a specification known as a xe2x80x9cburst mask.xe2x80x9d The burst mask specifies the rise time, fall time, duration, and power levels associated with the adjustable power control signal. The GSM signal consists of eight equal time slots. Each time slot must conform to the burst mask specification. Telephone software generates, by way of a digital-to-analog converter, an adjustable power control signal referred to as VRAMP. The ramp up time and ramp down time of VRAMP must conform to the shape of the burst mask. The amplitude of VRAMP dictates output power.
Yet another problem common to the prior art is that of inconsistent burst timing caused by input power fluctuations due to variations of temperature and power supply voltage. Burst timing is delayed with a decrease in input power and advanced with an increase in input power. Prior art systems use software to attempt to correct this problem, causing valuable code space to be consumed as a result.
Still another problem manifests itself in the prior art due to undesirable switching transients that occur when the up and down ramp of the burst is not smooth or changes shape. These switching transients also occur if the control slope of the amplifier has an inflection point within the output range, or if the slope is very steep. Consequently, it is difficult for a prior art system to change bias and gain in such a way as to prevent switching transients.
Thus, there remains a need for a power amplifier module with power loop control eliminating the need for traditional designs, which incorporate directional couplers, detector diodes, and power control ASICs, along with the problems associated with the employment of such devices.
The present invention provides for power amplifier control of amplifier circuitry including an input stage and one or more output stages. The input stage is powered separately from the output stage by a relatively fixed power source. The one or more output stages are supplied with power via a voltage regulator having a controllable output voltage. A closed loop control integrated with the amplifier stages forces the voltage output of the voltage regulator to track the profile of an adjustable control signal, such as VRAMP.
The closed loop control preferably includes an error amplifier, a feedback network, and a linear voltage regulator. The error amplifier compares a sample of regulator output voltage, scaled by the feedback network, with the voltage of the adjustable control signal. The voltage difference between the sampled voltage and the control voltage signal is amplified by the error amplifier and applied as a control signal for the voltage regulator. The control loop bandwidth is preferably high enough that for all practical purposes the voltage regulator instantaneously adjusts its output such that the voltage applied to the collector of the output amplifier stage corresponds to the control signal. Preferably, intermediate amplifier stages, which are series connected between the input and output stages, are supplied with power by way of the same voltage regulator controlled by the control loop.
Other aspects of the invention will become apparent to one skilled in the art upon a reading of the following detailed description of the invention, taken with the drawings.